Rupture of the anterior cruciate ligament (ACL) is relatively common and potentially crippling, affecting active people, both young and old. ACL injury may result in functional knee instability, secondary damage to other knee structures, and the early onset of degenerative joint disease. ACL reconstruction provides knee stability after ACL injury, however, it has not been shown to restore normal knee kinematics, and many aspects of ACL reconstruction and post-surgical rehabilitation remain problematic. Continued advancement depends on a more quantitative understanding of the forces and strains generated within the ACL during knee motion. The broad aim of this project is to determine the role of the human ACL during knee joint motion and under external loads. Using cadaveric knees, the in-situ force and strain distribution within the ACL will be measured with and without muscle stabilization. Data collected will then be used as a basis for evaluating the changes in knee kinematics and graft forces that result after ACL reconstruction. The effects of tension applied to the replacement graft at the time of surgery on these parameters will also be quantitatively examined. We will employ new technology involving a Universal Force-Moment Sensor (UFS) to determine the forces generated in the ACL under the application of known external loads. Complex paths of human knee motion in 6 degrees- of-freedom (DOF) will be recorded and then reproduced using an articulated robotic manipulator. Force/moment data coupled with reproducible knee motion will enable measurement of the in-situ forces within the ACL and ACL replacement grafts. Because this technology permits both position and force control of knee motion, the changes in knee kinematics resulting from ACL reconstruction can be directly examined. A 3-D strain analysis system will also be used to measure the strain distribution along the ACL. Information obtained in this study will have a direct bearing on the clinical management of ACL deficiency by providing data for surgical reconstruction. These findings will provide improved rationale for the selection of ACL replacement grafts, for the determination of appropriate reconstructive techniques, and for improvement of established rehabilitation protocols.